Process control systems utilize a variety of field devices to control process parameters. Fluid regulators are commonly distributed throughout process control systems to control the pressures of various fluids (e.g., liquids, gasses, etc.). Fluid regulators are typically used to regulate the pressure of a fluid to a lower and/or substantially constant value. Specifically, a fluid regulator has an inlet that typically receives a supply fluid at a relatively high pressure and provides a relatively lower and/or substantially constant pressure at an outlet.
As the high pressure process fluid travels through the process control system, the regulator reduces the pressure of the process fluid at one or more points to supply a process fluid having a lower or reduced pressure to a sub-system or other custody transfer points. For example, a gas regulator associated with a piece of equipment (e.g., a boiler) may receive a gas having a relatively high pressure from a gas distribution source and may regulate the gas to a lower, substantially constant pressure suitable for safe, efficient use by the equipment.
To prevent downstream pressure (i.e., outlet pressure) from reaching unsafe levels, fluid regulators typically include over pressure protection devices. Over pressure protection devices are operatively coupled to the fluid regulator and activate (e.g., when the downstream pressure of the fluid reaches a predetermined value) to prevent unwanted (e.g., unsafe) build-up of pressure at the downstream source. Some over pressure protection devices (e.g., shut-off devices) shut-off the flow of process fluid to the downstream source until the over pressure protection device is manually reset.
However, in some instances, an increase in fluid pressure due to, for example, temperature variations, may cause an undesired or unwanted activation of the over pressure protection device. To prevent unwanted activation of the over pressure protection device, some known spring-loaded fluid regulators may have an internal relief valve that is integrally formed with the fluid regulator. The internal relief valve bleeds process fluid to, for example, the atmosphere when the pressure of the process fluid increases due to, for example, temperature variations. If the downstream pressure of the process fluid exceeds the internal relief valve setting, the internal relief valve opens to bleed the fluid to the atmosphere. The over pressure protection device activates when the outlet pressure exceeds the predetermined pressure despite the process fluid bleeding through the internal relief valve. Fisher International series S201 and S202 are examples of such known spring-loaded regulators that include an internal relief valve.
Some known regulators, such as pressure-loaded regulators, do not vent to the atmosphere and, thus, an internal relief valve cannot be integrated with the regulator. In these known pressure-loaded regulators, an additional external relief valve is used to prevent unwanted activation or tripping of the over pressure protection devices due to, for example, temperature variations in the process fluid. However, such an external relief valve often requires additional installation labor, piping, manufacturing labor, inventory, maintenance, and, thus, increased costs.